There is a gear pump that sucks up fluid via an intake port using rotation of a pump rotor and discharges it to a discharge port. In a gear pump like this, when a rotational speed of the pump rotor increases, centrifugal action causes the fluid to easily flow in an outer peripheral side of the intake port and thus pressure in the outer peripheral side increases. On the other hand, the fluid does not easily flow in the inner peripheral side of the intake port, and pressure does not decrease. When a throttle becomes large immediately before an inter-teeth chamber is blocked from downstream edges of the intake port, cavitation easily occurs in the external teeth side. In a gear pump disclosed in U.S. Pat. No. 2,854,903, a bottom portion of the intake port is provided with a sloped bottom surface such that the intake port gradually becomes shallower from the upstream side to the downstream side of a fluid flow direction. Further, the sloped bottom surface is twisted three-dimensionally such that the intake port is deeper in the inner peripheral side which is closer to the central portion of the pump rotor and shallower in the outer peripheral side which is farther therefrom. Accordingly, the fluid is likely to flow in the inner peripheral side of the intake port, thereby preventing cavitation from occurring in the external teeth side of the inter-teeth chamber.
Meanwhile, a trochoid-type gear pump is disclosed in a gazette of Japanese Utility Model Registration No. 2588113. In this trochoid-type gear pump, in order to prevent cavitaion form occurring, a bottom portion of an intake port is provided with a sloped bottom surface such that the intake port becomes continuously shallower from the upstream side to the downstream side. Further, a shallow groove that runs continuously from the sloped bottom surface is formed at a downstream end portion of the intake port.
In the conventional gear pump disclosed in the aforementioned patent gazette, the sloped bottom surface provided on the bottom portion of the intake port is formed spiral from the upstream side to the downstream side in the fluid flow direction. Further, it needs to have a three-dimensionally twisted shape such that the intake port is deeper in the inner peripheral side which is closer to the central position of the pump rotor and shallower in the outer peripheral side which is farther therefrom. Therefore, design and manufacturing of the gear pump are complicated, and there is increase in cost.
Meanwhile, in the gear pump disclosed in the aforementioned gazette of the utility model, the downstream end portion of the intake port is provided with the shallow groove continuing from the sloped bottom surface so as to evenly cover an entire width of the intake port in the radial direction. Therefore, centrifugal action causes the fluid not to easily flow in the inner peripheral side of the intake port. It is not possible to solve the problem that pressure decreases in the external teeth side of the inter-teeth chamber, thereby cavitation easily occurring.
The present invention is devised in order to solve the aforementioned conventional problems, and an object thereof is to reliably prevent cavitation from occurring in an intake region of the gear pump with a simple structure.